Aqueous polymer dispersions as used for a host of applications, for example in the coatings and adhesive industries, etc., are required to be of high quality. One parameter of this quality is their keeping properties.
The keeping properties of dispersions are enhanced by the addition of preservatives and, in particular, by means of hygiene measures.
In storage it must be ensured in particular that the dispersion is not over-stored. Here, one of the critical factors is the way in which the storage tanks are filled and emptied.
Procedures to date have used the same line to fill and empty the storage tanks. If this line is attached in the base of the storage tank, the top layer of the dispersion wanders up and down in the course of the filling and removal operations, and becomes older and older, with all of the consequences of possible infestation by microorganisms as a result of excessive aging.
Another procedure uses a filling line which enters at the top of the storage tank and whose outlet is generally positioned toward the wall. The emptying line is judiciously located on the base of the storage tank. Although this procedure does some justice to the FIFO ("first in--first out") principle, filling from the top may lead to the formation of deposits on the walls and to unwanted foaming. Wall deposits are similarly undesirable because the deposit grows and can become the focus of nucleation. Foaming is undesirable because the foam readily dries and leads to the formation of gel specks (nonredispersible coagulum) in the product.
The problem of the filling and emptying of storage tanks and the storage of products which may be damaged by infestation with microorganisms is addressed in an article in the journal Farbe & Lack, Vol. 99, No. 1, 1993, pages 37-39 ff. The unwanted microbial growth is brought about in particular by the constituents of the dispersion and the water component, and the addition of preservatives--referred to as biocides--is recommended for reducing this growth.
In order to be able to store high-quality products reliably it is necessary to adopt the FIFO principle. This means that the parts of product first introduced into the storage tank are, in the course of subsequent removal, the first to be withdrawn, in order to avoid excessive aging of parts of the product.
In order to minimize these problems, stirred storage tanks have been proposed. Stirred storage tanks, however, are expensive and maintenance-intensive. Relatively large storage tanks whose contents are to be mixed by stirring must be provided with special reinforcement. Here too there may be instances of drying on the stirrer shaft or stirrer blades. Stirring may also introduce air into the dispersion, which is undesirable.